The present invention relates to a hydraulic control system for work vehicles, including a control unit connected to a first hydraulic work machine for controlling the steering wheels of the vehicle, a primary priority valve which by means of a first load-sensing system controls the pressure fluid flow to the control unit and the pressure fluid flow to at least one subsequent proportional valve, which by means of a second load-sensing system controls the pressure fluid flow to a connected secondary hydraulic work machine for controlling one of the work tools of the vehicle, and an open-centre valve connection to a tank and a fixed displacement pump.
Hydraulic control systems of this type are very widely used. These control systems are installed particularly in work vehicles like fork-lift trucks, tractors, agricultural, forestry and contractors's machines.
The control unit is provided with a steering wheel with which the driver of the work vehicle directly actuates the control unit mechanically when he wishes to change the movement of the work vehicle. By means of one or several control levers the driver can actuate the proportional valves of the system, thereby controlling the work tools of the vehicle when required. The transfer of signals from the control levers to the proportional valves may be mechanical, hydraulic, or electrical. The primary priority valve is designed to give the highest priority to the pressure fluid flow to the control unit. When the wheel is turned, the pressure fluid flow is divided in the priority valve, so that a sufficient flow of pressure fluid is continuously led to the control unit via the CF (control flow) port of the priority valve. The remaining flow of pressure fluid from the pump is available to the proportional valves via the EF (excess flow) port of the priority valve.
When neither the steering wheel nor the control levers to the proportional valves are actuated by the driver, the priority valve will be in a non-priority position, and the pressure fluid flow from the pump will flow to the tank via the open-centre valve connection. When the control unit is actuated via the steering wheel, the priority valve will control the distribution of pressure fluid via the first load-sensing system, so that as the main rule the control unit will always receive a sufficient flow of pressure fluid depending on the actual steering speed imparted by the driver through the wheel.
One problem of the hydraulic control systems of the type mentioned in the introduction is the fact, however, that the pressure fluid flow to the control unit ceases briefly at certain activation conditions when the control unit and one or several of the work tools of the work machine are operated simultaneously during the operation of the work vehicle, which will be explained in more detail in the following:
If one of the proportional valves is actuated via a control lever simultaneously with the wheel being turned to one extreme position, the load-sensing system of the relevant proportional valve will cause the open-centre valve connection to throttle down the tank connection, so that a pressure fluid flow can be led to the proportional valve being actuated. Hereby the pressure on the EF port of the priority valve will adjust to the pressure required by the proportional valve. It may be a case of the pressure at the CF port at the priority valve being much lower, depending on the pressure being required by the control unit from the pressure fluid flow supplied.
When the operation with the relevant work tool is finished, and the proportional valve is put in neutral position via the control lever, the pressure in the load-sensing system of the proportional valve will be relieved to the tank, whereby the open-centre valve connection will relieve immediately and open to the tank. Hereby the pressure on the EF port of the priority valve will drop off. Because the priority valve must react as a consequence of the opening to the tank of the open-centre valve, a time delay will occur, and a temporary "short circuit" will occur across the priority valve until it has had time to react. The "short-circuit" will cause the pump pressure and thereby the pressure at the CF port of the priority valve to drop off briefly (milliseconds). When the pressure at the CF port drops off, the control unit receives no pressure fluid flow, with the result that the wheel will stop briefly in its activation movement as a consequence of a moment peak. The driver of the work vehicle will feel this as a brief beat in the wheel, which is very annoying to the driver.
During operation of the work vehicles mentioned in the introduction with the hydraulic control system in question, many times during a work day the driver will actuate one or several of the work tools of the vehicle while at the same time operating the steering unit. The vehicle moves, and the driver wishes to change the path of the work vehicle. It may be a fork-lift truck, for instance, moving in a store, where the driver turns the wheel of the truck while at the same time activating a proportional valve controlling a hydraulic motor, which via a chain drive raises or lowers a load on the fork of the truck. When the load of the truck has been placed at the required height above the ground, the driver places the proportional valve in neutral position, whereby a brief beat will occur in the wheel, which will be particularly annoying to the driver.
For a number of years it has been attempted to redress the described problem, for instance by adapting the pressure build-up of the priority valve to the control system in question, but this has not at all been enough to remedy the problem.